Pre-tensionable bolts are previously known, which can be provided with a strain by means of a pressurized medium chamber arranged at one end of the bolt, after which it is possible to tighten a tightening member at the opposite end of the bolt. Such arrangements, for example, in connection with bolts intended for joining two elements, are presented e.g. in GB patent 1 382 192 and in DE publication 24 58 810. This latter publication also presents a traction means which is screwed around one end of a fixing means and comprises two parts, between which there is a space where pressurized medium is introduced and which traction means is, in the final joint, replaced with an ordinary nut.
Pre-tensioning arrangements of prior art have required a particular chamber structure which is screwed or joined by a quick coupling at the end of an elongated fixing means, such as a bolt. To perform the pre-tensioning, an axial bore is also needed inside the bolt as well as a power-transmitting rod placed therein, whose end forms a piston of the pressurized medium chamber which is acted upon to provide the pre-tensioning.
A simpler arrangement for providing the strain required by the pre-tensioning in an elongated fixing means for joining two elements is presented in the publication GB 2267944. This document discloses a tightening member, a “hydraulic nut”, which, on one hand, is fixed at the end of the elongated fixing means, provided with an outer threading, by means of a corresponding inner threading, and, on the other hand, abuts at its front surface on the surface of the first element to be joined. The tightening member is formed of three parts in such a way that the first part is fixed to the fixing means in a way transmitting power in the longitudinal direction, and the second part abuts at its front surface on one of the elements to be joined. The pressure of the pressurized medium can be introduced between the first and the second parts. The pressure is used to make the parts move farther away from each other, wherein the fixing means is stretched. The parts can be locked in this position by means of a third part which is effective therebetween and which is transferred to a locking position, after which the pressure can be removed.
In the arrangement presented in the publication, the second part which abuts the element to be joined, is locked to be axially immobile in relation to the first part by means of an annular locking part which is screwed by screw threads around the first part and which abuts, by its front surface facing the element to be joined, on the second part. The second part is thus placed underneath the first part and the locking part, joined together. So that the diameter of the tightening member could be reduced from the arrangement of prior art as presented in FIG. 1 of the GB document, the inner wall of the pressurized medium chamber in the first part is made thin and flexible elastically by the effect of the pressure in the chamber against the outer surface of the elongated fixing means.
Although the outer surface of the fixing means, according to the document, supports the inner wall against any further deflection caused by problems of tightness, this thinner inner wall is, however, the weak point in the structure and a potential cause of problems in the tightness for the pressurized medium.